IMPULSE RESPONSE TESTING USING WIDEBAND RF NOISELETS FOR STRUCTURAL AND MATERIALS CHARACTERIZATION AND DIAGNOSIS

PROJECT ABSTRACT We propose to develop a novel approach using radio frequency (RF) noiselets (i.e., noisy wavelets) for rapid, reliable, and robust determination of internal structural and material flaws caused by material degradation and corrosion in various types of military assets. Examples include aircraft skin, containers carrying weaponry and supplies, building walls, bridges, etc. Since RF signals are able to penetrate non-metallic dielectric materials with relatively low loss, depending on the material and the frequency used, they can be exploited for rapid non-point areal scanning of composite structures, joints, and interfaces. The proposed effort fits within the Materials Degradation/Corrosion and Structural Protection /Maintenance roadmaps described in the SBA NNR – Structures and Materials – Overview and Rollout briefing dated 12 June 2012. We propose the development and refinement of structural assessment using RF noiselets by characterizing the impulse response signatures of different types of materials and material structures. The entire dynamic behavior of a linear, time-invariant system in the time domain can be captured in its impulse response, i.e. the output of the system when excited by an impulse function at its input. Thus, impulse response testing provides a complete diagnosis of the system over its entire mode of operation. Since generating an ideal impulse function in time is quite difficult since realistic components have finite rise and fall times, an attractive alternative is to use ultrawideband (UWB) noise waveforms and use cross-correlation to extract the impulse response. Furthermore, cross-correlation is a useful approach provided that proper precautions are taken to recognize and include the effects of nonlinearities and to provide appropriate inputs which continually excite the system over its frequency range of interest. Noiselets are functions which are noise-like in the sense that they are totally uncompressible by orthogonal wavelet packet methods. Although noiselets are noise-like in the sense of being spread in time and frequency, there are patterns lurking in them. Noiselets are constructed via a multiscale iteration in exactly the same way as wavelet packets. Noiselets have also been shown to serve as excellent tools for multi-resolution analysis, based on their wide bandwidths. Since noiselet realizations are mutually incoherent, they can be employed as appropriate bases for compressive sampling. We will address the problem at hand using a combination of theoretical development, simulation studies, and experimental validation in our well-instrumented laboratory. During the first year, we will develop and refine the theoretical underpinnings of impulse response characterization of multilayered structures using UWB RF noiselets. During the second year, we will use the results of our first year's research to develop, test, and refine an impulse response testing system using UWB RF noiselets, and collect a comprehensive dataset on various materials and structures with and without defects to study the advantages and limitations of the approach. During the third and final year, we will finalize the design and testing of the impulse response measurement to characterize various structures and detect defects and anomalies using RF noiselets. We plan to interact closely with various Navy laboratories and related contractors in the conduct of this research. At the conclusion of this project, we will deliver to ONR a detailed project report describing a unique methodology for rapid, reliable, non-point, areal scanning of composite structures, joints, and interfaces.